Caption: DNA molecule and DNA autoradiogram, computer artwork. DNA (deoxyribonucleic acid) is composed of two strands twisted into a double helix. Each strand consists of a sugar-phosphate backbone (yellow) attached to a nucleotide base (blue and pink). There are four different bases: guanine, cytosine, thymine and adenine. A DNA autoradiogram shows the sequence of bases in a sample of DNA.